Evolution, 4th Edition

94 CHAPTER 4

selection is not efficient enough to drive mutation rates to zero. Were that to hap-

pen, life on Earth would be deprived of the fuel needed for adaptation, leading

inevitably to its extinction.

Fortunately, a small fraction of mutations are beneficial. These are the mutations

that spread by natural selection, and allow organisms to adapt. These changes are

therefore much more common as differences among species than they are among

new mutations that appear within a species. We will discuss many examples in the

next chapter.

A particularly dramatic type of mutation are homeotic mutations that transform

one body part into another (FIGURE 4.18). The effects of these mutations result

from the disruption of complex developmental pathways that evolved through a

large number of much smaller evolutionary changes (see Chapter 15). Mutations

do occasionally produce large beneficial effects, but there is no evidence that com-

plex structures (like the leg shown in Figure 4.18) originate by single mutations.

Goldschmidt’s idea of “hopeful monsters” that we discussed in Chapter 1 was an

evolutionary hypothesis that has been tested and proved wrong.

Germ line mutations and somatic mutations

Skin cancer is caused by mutations that result from exposure to sunlight. Those

mutations, however, cannot be transmitted to the next generation. Early in devel-

opment, many groups of animals set aside a small group of cells to form the germ

line, which then produces the gametes when the individual is sexually mature.

The rest of the cells in the early embryo go on to form the soma, consisting of all

the other tissues in the organism. These somatic cells leave no descendants to the

next generation. This explains why somatic mutations, like those responsible for

skin cancer, are not transmitted to the next generation.

Somatic mutations still have important evolutionary consequences in animals

that have a germ line. Cancer decreases an organism’s chance of surviving and

reproducing. Natural selection can favor mutations in the germ line that improve

DNA repair because they decrease the chance that mutations will occur in somatic

cells and produce a cancer.

Plants and some animals (such as sponges and corals) do not have a germ line.

Instead, gametes are made from somatic cells that divided many times as an indi-

vidual was growing. In these species, somatic mutations can be passed to a gamete

and then on to the next generation.

Is mutation Random?

The word “random” often appears with the word “mutation.” But are mutations in

fact random, and if so in what sense? This is an important question in evolution.

Futuyma Kirkpatrick But it is also a tricky one, because the word “random” has different meanings.Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_04.18.ai Date 11-03-2016

(A) Antenna (B) Leg

FIGURE 4.18 The homeotic mutation

Antennapedia in flies. (A) Head of a normal

Drosophila melanogaster. (B) Head of a fly

with the mutation, which converts the anten-

nae into legs.

04_EVOL4E_CH04.indd 94 3/23/17 8:55 AM